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Goals Understand the response of the detectors as a function of: –Operating conditions: Bias voltage Temperature Light intensity Temporal characteristics of the light input Provide measurements of electrical characteristics of the detectors as an input for the ASIC design Develop a procedure for the calibration of the response of the detectors: interpretation of the detector signals in terms of the incoming light intensity 4

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Categories of Observed Signals in Multipixel Devices 12 Single avalanches and afterpulses exist in single and multipixel detectors Double signal are specific to multipixel detectors

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Examples of Practical Questions Which detector to use (ignoring details like price and current availability) ? Hamamatsu/CPTA/IRST/SensL? Does is matter? What pixel size? 25/40/50/100 microns? How many pixels? What bias voltage to use? What temperature? Need to stabilize temperature and voltage? Or is it sufficient to read them back? Or change voltage with the changes of temperature? Need external calibration? Or is the single/nth pe peak sufficient to calibrate the gain? How to do the large scale quality control? Is the static (DC current) measurement sufficient? How long integration gate? How many time samples? In many instances the answer depends on the application. We are trying to collect data to serve as basis for such decisions. 13

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Samples We have samples of detectors from different sources. The results shown here are obtained with the samples of Hamamatsu MPPC. Why Hamamatsu? –You have to start somewhere –These detectors come with detailed detector-by-detector characterization. Very helpful to establish credibility of measurements We have samples of 25, 50 and 100 micron pixel devices. Comparison of their characteristics provides a good test of out Standard Model. Most of the detailed studies, so far, carried out for 25 micron devices. Why? –Most attractive for the calorimetric/pulse height measurement applications (dynamic range!) –Probably most challenging case 14

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Cross Talk Rates as a Function of Bias Voltage Cross talk probability increases with the bias voltage Cross talk probability is bigger for larger size pixels But… The cross talk is mediated by infrared photons produced in the avalanche, hence is ought to be proportional to the gain. And different size pixel detectors have different gain !

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Cross Talk Probability as a Function of Gain At the same gain the cross-talk probability is much larger for smaller size pixels At the operating point the Hamamatsu detectors have very small cross talk (~few %)

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Cross-talk Probability as a Function of Avalanches Naïve expectations: with two avalanches present the number of photons is doubled, hence the cross talk probability ought to be higher Ditto for three avalanches present Naïve model doesn’t hold: some conspiracy between the solid angle and the photons mean free path??

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Time Difference Between Dark Pulses V bias =72.75 V Fraction of traces with exactly one pulse: Expected : 0.39 Observed: 0.08 Large number of pulses closely clustered in time

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‘Dark’ Rates vs Voltage ‘Raw’ rate ‘True’ rate ‘True’ rate, single peak method (inferior)  Rate of ‘true’ dark counts increases slightly with bias voltage(reflecting the increase of the probability that a free electron will start an avalanche). This is expected as the rate of free carrier generation depends on the temperature and not the bias voltage.  Observed exponential growth of the dark rate is caused by afterpulsing  At the higher bias voltage ‘dark’ pulses come in clusters Probability that a single avalanche will induce at least one more avalanche (afterpulse)

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Single (Isolated) Dark Pulses: Self- Calibration of the Detector With longer gate or higher voltage a long tail (afterpulses) and a double avalanche peak (cross talk) appear

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Dark Counts: Comment About the Rates 71.5 V, integration gate of 50 nsec Dark count rate: what is the reduction when cutting at 1.5 pe?? It depends on the definition of ‘rate’: –Factor of (cross talk probability) if measure the amplitude, bias voltage dependent –Factor of 5-10 if measure integral within some gate (gate dependent), dominated by afterpusling

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Laser Light Pulses Fractional content of the ‘zero’ bin -> average number of photons detected 3-5 photons detected Good agreement between ‘charge’ and ‘amplitude’ –based measurement An apparent increase of the laser intensity with the bias voltage is an indication of the increase of the PDE (avalanche initiation probability) by a factor of ~ 1.5 between 71 V and V

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Light-Induced vs Dark Pulses? 39 Single p.e. pulses before during and after the laser gate recorded at different bias voltages: pulse shape does not depend on the avalanche origin pulse shape does not depend on the voltage, apart from the overall factor (gain)

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Analysis of the ‘Laser Gate’ Data Two possible measures of the signal/readout strategies: – the peak amplitude (peak sensing readout) Practical for very short light pulses (lasers, Cherenkov) –Integrate the charge within some gate (30 nsec shown thereafter) More typical case for HEP (like calorimetry) Gate length? Number of samples?

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Reconstructing the Poisson Distribution (Charge and Amplitude)

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Laser Pulses vs Bias Voltage: Amplitude Notice the decrease of the number of zero’s and the general shift to the right: increase of the mean number of detected photons as a result of the increase of detection efficiency with bias voltage. Consistent with the measurement using ‘zeros’.

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Charge of the laser pulse in 30 nsec gate With the increasing bias voltage afterpulses increase the response, but degrade the ability to detect individual avalanches. Poisson shape destroyed This is caused by additional pulses or parts of thereof sneaking into the integration gate.

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Charge of the Laser pulse in 10 nsec gate with afterpulse veto Require that [Q(30) Q(10)]<0.15xQ(10), i.e. no afterpulse immediately following the laser pulse. Ability to count individual avalanches restored. This is not a very practical solution in real life applications, though. It may be, perhaps, of some use in situations where: Arrival time of the light pulse is known (timing of the gate) Input light pulse has small duration (~ 1-2 nsec)

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Output Pulse Shape as a Function of Bias Voltage Average pulse shape of the response to the laser light as a function of the bias voltage (red – Vbias =71 V, blue – Vbias = V) Clear evidence for afterpulsing component growing with the voltage making pulses bigger and longer.

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Variation of ‘observables’ with Bias Voltage Different measures of the signal show different variation with the bias voltege (at fixed temperature and the same light signal). For 1.5 V variation of the bias voltage the peak amplitude grows by a factor of about 2.5, whereas the integral of charge in 100 nsec gate changes by a factor of 7 Need to keep the voltage (and temperature) very stable or need to devise a precise calibration procedure.

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Choosing the Gate for the Readout? Observed signal grows with the bias voltage. This growth has several components: increase of the gain increase of afterpulsing. The latter is a much bigger effect. So what?? Afterpulses provide a kind of additional gain. True, but this contribution fluctuates  degrades the charge measurement resolution (excess noise factor). Relative width of the observed pulse height spectrum slightly decreases with bias voltage for 10 nsec gate (presumably a reflection of the increased number of detected photons), but it increases for longer gates. Bottom plot shows a contribution to resolution from fluctuations of the afterpulses contribution in different gates.

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Summary We are completing a general test facility Detailed studies of various aspects of the response of the PPD’s allow good understanding of the detectors behaviour Hamamatsu detectors (25 microns) have relatively low dark noise rates They have very short recovery time (5 nsec) (Owing to a short recovery time) Response of the detectors is dominated at higher bias voltages by the afterpulsing Cross talk is at the few percent level and it is always much smaller than afterpulsing 54